In 2012, Notch developed a JavaScript version of Minecraft 4k. This version generates textures slightly differently to the original Java version, so this project aims to provide a means to view & save these textures.
The comments for the texture generation algorithm were taken from this version of Minecraft 4k.
The main difference is the Java version uses a set seed to always generate the same textures, but the PRNG in JavaScript cannot be seeded so the textures are randomly generated.
The algorithm doesn't assign a value to the first 768 elements (3 16px × 16px textures) of the texmap array so when they are accessed, their value is undefined. The rendering code in Minecraft 4k uses bitwise operators on the values from the texmap array. Bitwise operators convert operands to 32-bit integers so undefined is converted to 0. This means the first three textures are entirely black (0x000000). Black is considered transparent in the rendering code, so these textures are rendered as air blocks.
Pseudocode to illustrate this (ray casting):
if (color > 0) then
draw pixel
next ray
else
step ray
endif
Therefore, for completeness, these textures are included and rendered as transparent.